In recent years, non-linear optical materials, i.e., materials having non-linearity between polarization and electric field developed when given a strong light field such as laser light have drawn public attention.
Such materials are generally known as non-linear optical materials and are described in detail in, for example, "Nonlinear Optical Properties of Organic and Polymeric Material", ACS SYMPOSIUM SERIES 233, edited by David J. Williams, American Chemical Society, 1983, Masao Kato and Hachiro Nakanishi, "Organic Nonlinear Optical Materials", C.M.C, 1985, and D. S. Chemla and J. Zyss, "Nonlinear Optical Properties of Organic Molecules and Crystals", vols. 1 and 2, Academic Press, 1987.
One of the applications of such a non-linear optical material is a wavelength conversion device using the second harmonic generation (SHG), the addition of frequencies (or sum frequency) and the subtraction of frequencies (or differential frequency) on the basis of a secondary non-linear effect. As such non-linear optical materials there have been put into practical use inorganic perovskites such as lithium niobate. However, .pi. electron-conjugated organic compounds containing electron donative groups and electron attractive groups have been known to far surpass the above mentioned inorganic substances in many non-linear optical properties.
In order to form a non-linear optical material with a higher performance, it is necessary to arrange a compound having a high non-linear susceptibility in molecular state in such a manner that no inverse symmetry occurs. It has been known that a compound having a long .pi. electron conjugate chain is useful for the development of a high non-linear susceptibility, which is one of the many non-linear optical properties. Various examples of such a compound are described in the above cited references. As can be seen by those skilled in the art, these compounds exhibit an absorption peak shifted in the long wavelength range. In particular, these compounds are subject to drop in the blue light transmittance which impedes the generation of blue light as second harmonic. This is true in the case of p-nitroaniline derivatives. The fact that the effficiency of the second harmonic generation greatly depends on the transmittance of the second harmonic can be proved by Alain Azema, "Proceedings of EPIE", vol. 400, Now Optical Materials, 1983, FIG. 4, page 186.
Thus, the apperance of a non-linear optical material which exhibits a high transmittance of blue light has been desired. It has been heretofore proposed to replace carbon atoms in benzene nucleus of nitroanline by nitrogen atoms or the like. However, this approach has not necessarily given satisfactory results.
On the other hand, better approaches have been disclosed in JP-A-62-210430 and JP-A-62-210432 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
Furthermore, many materials are disclosed in JP-A-62-59934, JP-A 63-23136, JP-A-63-26638, JP-B-63-31768 (the term "JP B" as used herein means an "examined Japanese patent publication"), JP-A-63-163827, JP-A 63-146025, JP-A-63-85526, JP A-63-239427, JP-A-1-100521, JP-A-64-56425, JP-A-1-102529, JP-A-1-102530, JP-A-1-237625, and JP-A-1-207724.
As previously mentioned, such a material should not only be satisfied in properties in molecular state but also essentially cause no inverse symmetry in molecular arrangement in aggregated state so that it can be effectively used as secondary non-linear optical material. However, it is extremely difficult in the art to expect such a molecular arrangement. Further, the probability of occurrence of such a molecular arrangement is not so high.
If such a material is used as device for the conversion of wavelength, it is necessary to thoroughly consider the molecular arrangement in the crystal. Many of the above mentioned materials are not necessarily considered thoroughly in this respect. Further, to date, no wavelength conversion devices using organic non-linear optical materials have ever appeared commercially.
The reason for this phenomenon can be thought as follows:
When the above mentioned non-linear optical material is used to form a fiber type light wavelength conversion device, the crystal is not oriented in such a direction that the maximum non-linear optical constant of the material can be utilized. After all, the light wavelength conversion device thus obtained doesn't exhibit so high a wavelength conversion efficiency.
Further, the longer the light wavelength conversion device is, the higher is the wavelength conversion efficiency attained thereby. However, the above mentioned material is difficult to form a homogeneous single crystal. Thus, such a material is also disadvantageous in that it is not suitable for the preparation of a long light wavelength conversion device.